This invention relates to processes for the purification of aromatic compounds, particularly bisphenols.
Bisphenols are valuable compounds useful in the preparation of epoxy resins and polycarbonates. High quality epoxy resins, and particularly polycarbonates, require especially pure bisphenols for use in their preparation. Known processes for the production of bisphenols produce bisphenols having certain impurities including isomers, analogs and homologs. Therefore, it is necessary to have processes for the purification of the crude product. The purification of the crude product often proves difficult since the common impurities frequently have characteristics very similar to the desired compound.
Numerous processes exist for the purification of bisphenols in general and particularly for the purification of p,p'-isopropylidenediphenol. U.S. Pat. No. 3,919,330 discloses a purification process wherein crude isopropylidenediphenol crystals are dissolved in an organic solvent. Water is then added, the solution is cooled, p,p'-isopropylidenediphenol crystallizes and the crystals are then separated by filtration or centrifugation. Another process is described in U.S. Pat. No. 4,354,046 where it is taught to crystallize a bisphenol from a single liquid phase comprising water, the bisphenol and an organic solvent. The mother liquor is stripped of solvent and water and mixed with phenol. A cation-exchange resin is used to convert the impurities to the desired product and the phenol is recycled. It is also known to purify p,p'-isopropylidenediphenolby using an aqueous alkaline solution (U.S. Pat. No. 4,507,509). Another process for the purification of bisphenols is taught in U.S. Pat. No. 2,959,622 where it is taught to wash the crude bisphenol with water, neutralize it with an alkaline solution and dissolve the crystals in hot water and an immiscible organic solvent. The organic phase is then cooled to recover the product. U.S. Pat. No. 4,461,915 teaches that p,p'-isopropylidenediphenol may be purified by mixing the molten isopropylidenediphenol with water, crystallizing the mixture and then washing the crystals with an organic solvent.
Other methods of purifying bisphenols include contacting bisphenol crystals in water with an organic washing solvent in a continuous, multi-stage, counter-current extraction column (U.S. Pat. No 4,447,655). A method of purification of a multi-component molten mixture using a countercurrent, cooling multi-stage crystallizer/purifier is described in European Patent Application No. 0 105 524A2.
In each of these processes for the purification of bisphenols, a major goal is to produce bisphenols of high purity in high yields. However, these goals are not always met and it is frequently necessary to sacrifice high yields to obtain high purity or vice versa. In addition, many of the existing methods for the purification of bisphenols require the use of organic solvents which may be flammable or toxic. What is needed is a method for obtaining high purity bisphenols in high yields while avoiding the use of organic solvents.